1. Field of the Invention
The present invention relates to techniques for use in an Orthogonal Frequency Division Multiplexing (OFDM) wireless communication system. More particularly, the present invention relates to techniques for estimating a channel and interference in an OFDM wireless communication system with multiple antennas.
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier technique that is widely used as an access technique in a modern wireless communication system due to its robustness to multipath fading and simple implementation. The number of OFDM subcarriers, including guard subcarriers, in an OFDM wireless communication system is typically selected as power of two, which allows for the use of a Fast Fourier Transform (FFT) algorithm during reception and an Inverse FFT (IFFT) algorithm during transmission. An example of OFDM transmission is described below with reference to FIG. 1.
FIG. 1 illustrates an OFDM transmitter according to the related art.
Referring to FIG. 1, the OFDM transmitter 100 includes an IFFT 102, a Parallel to Serial (P/S) converter 104, a Cyclic Prefix (CP) inserter 106, a Digital to Analog Convertor (DAC)/Radio Frequency (RF) up-converter 108, Power Amplifier (PA) 110, and at least one Transmit Antenna 112. Complex modulations symbols X(k) where k=0, 1, . . . , (N−1) and guard subcarriers are mapped to the input of IFFT 102. No information may be transmitted on the guard subcarriers. After the IFFT operation, the resulting information is serialized by P/S converter 104. A cyclic prefix is added after the serialization by the CP inserter 106. The resulting sequence is digitized and up-converted into RF by the DAC/RF up-converter 108, amplified by the PA 110 and transmitted using the Transmit Antenna 112.
An example of OFDM reception is described below with reference to FIG. 2.
FIG. 2 illustrates an OFDM receiver according to the related art.
Referring to FIG. 2, the OFDM receiver 200 includes at least one Receive Antenna 202, a Low Noise Amplifier (LNA) 204, an Analog to Digital Convertor (ADC)/RF down-converter 206, a CP remover 208, a Serial to Parallel (S/P) converter 210, an FFT 212, and a Frequency-Domain Equalization (FDE) operation 214. A signal received via the Receive Antenna 202 is low noise amplified by LNA 204. The resulting signal is down-converted from RF and converted from digital into analog by ADC/RF down-converter 206. The CP samples are discarded by the CP remover 208 and the resulting signal is converted into parallel by the S/P converter 210. An FFT operation is performed on the received samples sequence by the FFT 212. A FDE operation is performed by FDE 214 using channel estimates obtained from received pilots or reference signals. Thereby, the estimates of the transmitted complex modulation symbols are obtained.
A typical cellular wireless communication system includes a collection of fixed Base Stations (BSs) that define coverage areas or cells. Typically, a Non-Line-Of-Sight (NLOS) radio propagation path exists between a BS and a Mobile Station (MS) due to natural and man-made objects that are situated between the BS and the MS. As a consequence, the radio waves propagate via reflections, diffractions and scattering. The waves arriving at the MS in the DL direction (at the BS in the UpLink (UL) direction) experience constructive and destructive additions because of different phases of the individual waves. This is due the fact that, at the high carrier frequencies typically used in the cellular wireless communication system, small changes in the differential propagation delays introduces large changes in the phases of the individual waves. If the MS is moving or there are changes in the scattering environment, then the spatial variations in the amplitude and phase of the composite received signal will manifest themselves as time variations known as Rayleigh fading or fast fading. The time-varying nature of the wireless channel requires a very high Signal-to-Noise Ratio (SNR) in order to provide a desired bit error rate or packet error reliability.
Multiple Input Multiple Output (MIMO) schemes use multiple transmit antennas and multiple receive antennas to improve the capacity and reliability of a wireless communication channel. A wireless communication system implementing the MIMO scheme (hereafter referred to as a MIMO wireless communication system) theoretically enables a linear increase in capacity of K, where K is the minimum of the number of transmit (M) and receive (N) antennas (i.e., K=min(M, N)). A simplified example of a 4×4 MIMO wireless communication system is described below with reference to FIG. 3.
FIG. 3 illustrates an example of a 4×4 MIMO wireless communication system according to the related art.
Referring to FIG. 3, the 4×4 MIMO wireless communication system 300 includes a transmitter 310 and a receiver 320. The transmitter 310 includes a precoding unit 312 that receives four different data streams Layers 1-4 that are transmitted separately from the four transmit antennas TX1-TX4. The receiver 320 includes a spatial processor 322 that receives the signals transmitted by the transmitter 310 via four receive antennas RX1-RX2. The spatial processor 322 performs spatial signal processing on the received signals, such as Minimum Mean Squared Error (MMSE) spatial filtering, MMSE-Soft Interference Cancellation (SIC) spatial filtering or Maximum Likelihood (ML) decoding, in order to recover the four data streams Layers 1-4.
The MIMO channel estimation includes estimating the channel gain and phase information for links from each of the transmit antennas to each of the receive antennas. Therefore, the channel for an M×N MIMO wireless communication system consists of an N×M matrix:
                    H        =                  [                                                                      h                  11                                                                              h                  12                                                            …                                                              h                                      1                    ⁢                    M                                                                                                                        h                  21                                                                              h                  22                                                            …                                                              H                                      2                    ⁢                    M                                                                                                      ⋮                                            ⋮                                            …                                            ⋮                                                                                      h                                      N                    ⁢                                                                                  ⁢                    1                                                                                                h                                      M                    ⁢                                                                                  ⁢                    2                                                                              …                                                              h                  NM                                                              ]                                    Equation        ⁢                                  ⁢                  (          1          )                    where hij represents the channel gain from transmit antenna j to receive antenna i. In order to enable the estimations of the elements of the MIMO channel matrix, separate pilots are transmitted from each of the transmit antennas.
An example of a single-user MIMO wireless communication system is described below with reference to FIG. 4.
FIG. 4 illustrates a single-user MIMO wireless communication system according to the related art.
Referring to FIG. 4, a BS 402, an MS-1 404, and an MS-2 406 are shown. Here, it is assumed that BS 402 will only be transmitting to MS-2 406. In this case, all of the MIMO layers in the cell Layers 1 and 2 are transmitted to MS-2 406.
An example of a multi-user MIMO wireless communication system is described below with reference to FIG. 5.
FIG. 5 illustrates a multi-user MIMO wireless communication system according to the related art.
Referring to FIG. 5, a BS 502, an MS-1 504, and an MS-2 506 are shown. Here, it is assumed that BS 502 will be transmitting to MS-1 504 and MS-2 506. In this case, the MIMO layers Layers 1 and 2 in the cell of BS 502 are shared among MS-1 504 and MS-2 506.
An example of Frequency Division Duplex (FDD) is described below with reference to FIG. 6.
FIG. 6 illustrates an FDD frame according to the related art.
Referring to FIG. 6, FDD frame 600 includes DownLink (DL) 602 and UL 604 transmissions that occur simultaneously on deferent frequency bands. The FDD frame is divided into timeslots referred to as subframes.
An example of Time Division Duplex (TDD) is described below with reference to FIG. 7.
FIG. 7 illustrates TDD frames according to the related art.
Referring to FIG. 7, a TDD frame 700 is shown that use a single frequency band for DL 702 and UL 704 transmissions with a 4:4 (four subframes for DL and four subframes for UL) configuration. Also, a TDD frame 710 is shown that uses a single frequency band for DL 712 and UL 714 transmissions with a 6:2 (six subframes for DL and two subframes for UL) configuration. Similar to the FDD frame, a TDD frame is divided into timeslots referred to as subframes. While two specific examples of a TDD frame configuration are shown, the transmission time may be shared between DL and UL transmissions in other proportions. An advantage of the implementation of TDD in a wireless communication system is that UL and DL channels are symmetric, which allows for DL channel quality and MIMO channel estimation at a BS from UL transmissions. When FDD is implemented in a wireless communication system, an MS calculates channel quality and MIMO information from DL pilot transmissions, which is feed back to the BS on a feedback channel.
In an OFDM wireless communication system, a subframe is divided in the frequency domain into different Resource Blocks (RBs). A RB consists of multiple subcarriers and OFDM symbols. A RB is considered as minimum unit of resource allocation for a user. An example of an OFDM RB is described below with reference to FIG. 8.
FIG. 8 illustrates an OFDM RB according to the related art.
Referring to FIG. 8, 18 subcarriers and six OFDM symbols form one RB. Of course, RBs may be formed using differing numbers of subcarriers or OFDM symbols. Typically, training or pilot signals will be transmitted among payload data in the RB.
Training signal or pilot overhead is a significant concern in a MIMO wireless communications system because separate pilot signals are required for each of the transmit antennas. An example of pilot overhead in a MIMO wireless communications system is described below with reference to FIG. 9.
FIG. 9 illustrates Channel Quality Indication (CQI) and Precoding Matrix Indication (PMI) feedback according to the related art.
Referring to FIG. 9, pilot signals 902-1, 902-2, . . . , 902-M are transmitted from BS 910 to MS 920. The pilot signals 902-1, 902-2, . . . , 902-M for different antennas may be made orthogonal in time, frequency or code-domain. The pilot signals 902-1, 902-2, . . . , 902-M are used by the MS 920, among other things, for Channel Quality Indication CQI and PMI calculation 922. This information is then fed back to the BS 910 in a CGI/PMI feedback message 904. The BS 910 makes use of this information in scheduling decisions as well as MIMO, modulation and coding format selection for the MS 920.
The pilot signals 902-1, 902-2, . . . , 902-M used for CQI and PMI calculation are typically referred to as common pilot signals as they are used by all MSs in a cell. The common pilot signals are generally not precoded since MSs use these signals as a reference for PMI calculation. The common pilot signals can also be used for data demodulation. However, for data demodulation, precoded dedicated signals are generally considered more useful because the channel estimation performance may be improved due to precoding gain on the pilot signals. The dedicated pilot signals are targeted for a desired MS and cannot be used by other MSs in the cell as a reference because the dedicated pilot signals are precoded with an MS specific precoding vector or matrix. The dedicated pilot or reference signals also result in smaller overhead because the number of pilot signals required is equal to the number of MIMO layers transmitted, which may be smaller than the total number of transmit antennas in the system due to MIMO rank adaptation.